This invention relates to an oil control piston ring assembly, and more particularly to an oil control piston ring assembly which includes at least one rail ring and a rail ring expander.
Oil control piston ring assemblies include a rail ring and an expander and are positioned within a piston ring groove. The expander positions the rail ring and resiliently urges it into engagement with an adjacent cylinder wall. Examples of assemblies of rail rings and expanders are illustrated in U.S. Pat. Nos. 2,833,604; 3,172,672; 3,355,180; 3,477,732; 3,628,800; 3,741,569; and 3,762,728. In some expanders, the expander spring force is created by circumferentially spaced spring fingers which deflect radially and apply a radially directed, resilient force to the rail ring. In other expanders, the resilient force is created by circumferential or hoop stresses in the ring which resist circumferential contraction of the expander.
Many oil control piston ring assemblies include a pair of rail rings and an expander. If the expander is of the type which circumferentially contracts and expands in response to dimensional changes such as wear of the rail rings or a varying cylinder diameter, a loss of oil control can occur. For example, this loss of oil control can occur when one rail ring applies a force to the expander to cause contraction of the expander and, as a result, the other rail rings loses its loading on the cylinder wall. Only one rail ring is then functioning to provide oil control when two rail rings should be functioning, and thus oil control is detrimentally affected.
Oil control piston ring assemblies having expanders with spring fingers which apply a radially directed spring force to a pair of rail rings also are subject to loss of oil control. In such assemblies, the rail rings can contract independently of each other. Thus, loss of oil control, as occurs when circumferentially contractible expanders are used, is avoided. However, the spring fingers have a relatively high spring rate. Thus, as the deflection of the spring fingers diminishes due to wear of the rail rings, for example, the radially directed force applied by the spring fingers is significantly reduced. The force reduction reduces the unit pressure between the rail ring and the cylinder wall, which permits oil to pass more easily between the ring and the wall. Thus, oil control is significantly detrimentally affected.
One of the patents listed above, namely U.S. Pat. No. 3,628,800, describes and illustrates an expander ring which is circumferentially expandable and contractible to supply a first resilient force urging portions of the expander ring itself into contact with the cylinder wall and which includes spring fingers that deflect radially to supply a second resilient force urging a rail ring into engagement with the cylinder wall. The two resilient forces function in an independent manner. The first force causes direct engagement between the wiping portions of the expander and the cylinder wall, while the second force acts on the rail ring. The first force normally does not affect the force on the rail ring, which is determined solely by the spring fingers.